The invention relates to maraging steel having high fatigue strength which is suitably used for members, which are required to have high fatigue strength, such as a power transmission belt etc. used in a continuously variable transmission of an automobile etc., and maraging steel strip formed of the maraging steel.
Hitherto, since conventional maraging steel has very high tensile strength of about 2000 MPa, it is used for forming members, which are required to have high strength, such as members for rockets, members for a centrifugal separator, members for aircraft, members for a continuously variable transmission of an automobile engine, dies and etc. The representative composition of the maraging steel is, , for example, 18% Ni-8% Co-5% Mo-0.4% Ti-0.1% Al-bal. Fe. The maraging steel contains, as strengthening elements, appropriate amount of each of Mo and Ti, so that the maraging steel can obtain high strength which is achieved by such an aging treatment as to precipitate intermetallic compounds such as Ni3Mo, Ni3Ti, Fe2Mo etc.
In the conventional maraging steel, a very high tensile strength can be obtained, however, the fatigue strength thereof is not necessarily high. In general, fatigue strength has such a tendency as to be raised in proportion to the increase of the hardness and tensile strength, however, in a high strength material having hardness not less than about 400 Hv and tensile strength not less than about 1200 MPa, the fatigue strength does not increase even in a case where both of the hardness and the tensile strength increase. This is also applicable to the conventional maraging steel. Thus, there has been desired a novel maraging steel in which a higher fatigue strength can be obtained.
Further, since the conventional maraging steel usually contains a large amount of Co which is an expensive element, it becomes very expensive, and new maraging steel low in price has been desired.
The object of the invention is to provide new maraging steel having high fatigue strength which is low in production cost, and to provide maraging steel strip made of the new maraging steel.
In conventional high strength steel such as the conventional maraging steel explained above, it is known that the fatigue fracture is initiated by cracks occurring in and propagating from the surface thereof in a case where the fatigue fracture occurs in a low cycle range, as disclosed in Japan Mechanical Society Theses Vol. A64, pages 2536 to 2541. Further, it is also known that, in another case of a very high cycle range exceeding 107 cycles which is deemed to be the fatigue limit, the fatigue fracture of the steel is not initiated from the surface thereof but is initiated from inclusions included in the steel.
In the course of researching new maraging steel, the inventors of the invention have noticed such basic, technical concept as fatigue strength relating to the fatigue fracture initiated from the surface of the steel can be improved by providing compressive residual stress in the surface and as fatigue strength relating to the fatigue fracture initiated from the interior of the steel can be improved by making the inclusions fine in size.
As the result of further intensive research for obtaining new maraging steel hitherto desired, the inventors of the invention have found out that, in order to enhance the fatigue strength relating to the fatigue fracture initiated from the surface of the steel, it is effective to perform proper nitriding so that large, compressive residual stress may occur in the surface of the steel.
Further, as the results of the detailed research regarding the initiation of the fatigue fracture occurring from the interior of the conventional maraging steel, the inventors of the invention have found out that the fatigue fracture is initiated from inclusions and that the inclusions are TiN (or Ti(C, N)), so that it is noticed that non-existence of TiN (or Ti(C, N)) in the steel is effective to enhance the fatigue strength. In order to make TiN non-existent, it is effective to reduce Ti or N (nitrogen), however, extreme decrease of N is difficult insofar as melting apparatus of mass production is concerned, and causes such a problem as the production cost is raised greatly.
On the other hand, it is deemed that, by greatly reducing the amount of Ti, it becomes possible to reduce TiN and to make TiN fine in grain size. However, Ti is one of the important strengthening elements of the maraging steel, and the strength thereof is greatly lowered in a case where the amount of Ti is simply reduced. This maraging steel in which the content of Ti is reduced is disclosed in each of JP-A-10-152759 entitled xe2x80x9cMaraging Steel Superior in Toughnessxe2x80x9d and JP-A-1-142052 entitled xe2x80x9cSeamless Metallic Belt and Production Method of Samexe2x80x9d.
In the maraging steel of JP-A-10-152759, however, N (nitrogen) in the range of 0.005 to 0.03% is rather added positively. In the seamless, metallic belt of JP-A-1-142052, the content of Co is in the range of 8 to 15% which is in the same level as that of conventional maraging steel containing Ti, so that the production cost thereof is high unfavorably.
The inventors of the invention have fount out the first technical matter that, in new maraging steel in which the amount of each of Ti and N contained therein is restrained to be in a low level and in which the amount of Co contained therein is made to be in a low level for making the production cost thereof low, the decrease in tensile strength due to the lowering of the amount of each of Ti and Co can be compensated by adding a small amount of each of Si, Mn, Al and etc. under such a condition as to limit the value of (3Si+1.8Mn+Co/3+Mo+2.6Ti+4Al) to be in an appropriate range. Further, the inventors of the invention have further found out the second technical matter that, by simultaneously adding both of a small amount of B (boron) and an appropriate amount of Nb, Ta, and/or W, it becomes possible to make the prior austenite grains of the low Ti, low Co maraging steel fine in size which steel contains Si, Mn, Mo and etc., which is effective to enhance the tensile strength and the fatigue strength thereof.
In addition, the inventors have found out the third technical matter that, although the amount of Ti does not cause large influence on the surface hardness measured after the nitriding, the absolute value of surface compressive residual stress becomes large when the amount of Ti is in a low level. Furthermore, the inventors have found out the fourth technical matter that, by adding an appropriate amount of Cr, it becomes possible to increase the absolute value of the surface compressive residual stress occurring through the nitriding. By combining these technical matters, the inventor succeeded in achieving the invention.
Namely, according to the first aspect of the invention, there is provided a maraging steel having high fatigue strength, characterized by consisting, by mass, of not more than 0.008% C, from 0 inclusive but not more than 2.0% Si, from 0 inclusive but not more than 3.0% Mn, not more than 0.010% P, not more than 0.005% S, 12 to 22% Ni, 3.0 to 7.0% Mo, less than 7.0% Co, not more than 0.1% Ti, not more than 2.0% Al, less than 0.005% N (nitrogen), not more than 0.003% O (oxygen), and the balance substantially Fe, the total amount of 3Si+1.8Mn+Co/3+Mo+2.6Ti+4Al being in a range of 8.0 to 13.0%.
According to the second aspect of the invention, there is provided a maraging steel having high fatigue strength, characterized by consisting, by mass, of not more than 0.008% C, from 0 inclusive but not more than 1.0% Si, from 0 inclusive but not more than 2.0% Mn, not more than 0.010% P, not more than 0.005% S, 12 to 22% Ni, 3.0 to 7.0% Mo, less than 7.0% Co, not more than 0.05% Ti, not more than 2.0% Al, less than 0.005% N (nitrogen), not more than 0.003% O (oxygen), and the balance substantially Fe, the total amount of 3Si+1.8Mn+Co/3+Mo+2.6Ti+4Al being in a range of 8.0 to 13.0%.
According to the third aspect of the invention, there is provided a maraging steel according to any one of the first and second aspects of the invention, further containing not more than 4 mass % Cr. According to the fourth aspect of the invention, there is provided a maraging steel according to any one of the first to third aspects of the invention, characterized by further containing not more than 0.01 mass % B. According to the fifth aspect of the invention, there is provided a maraging steel according to any one of the first to fourth aspects of the invention, characterized by further containing, by mass, at least one kind selected from the group consisting of not more than 1.0% Nb, not more than 2.0% Ta, and not more than 2.0% W. According to the sixth aspect of the invention, there is provided a maraging steel according to any one of the first to fifth aspects of the invention, characterized by further containing, by mass, at least one kind not more than 0.5% in total selected from the group consisting of Nb, Ta, and W.
Further, the maraging steel according to any one of the first to sixth aspect of the invention may be formed so that it has such prior austenite grains fine in size as to be not less than 9 in ASTM number, which is the seventh aspect of the invention. Furthermore, the maraging steel strip made of the maraging steel according to any one of the first to seventh aspect of the invention may be provided with a nitride layer formed on a surface portion thereof so that it has compressive residual stress on the surface thereof, which is the eighth aspect of the invention.